A conventional pin tumbler mechanism generally has a rotatable inner cylinder for receiving a key inserted into a key hole in the inner cylinder to move pins so that separating lines of the pins can come in line with a contact line of the inner cylinder and an outer cylinder, enabling the inner cylinder to be rotated to unlock a lock. This structure can be illegally pricked open by a very sharp pointed tool, which is inserted in the key hole to move the pins one by one in order. So this kind of lock has a low safety against illegal breakage.